Tape-based storage devices are typically used for archival storage of data stored on hard drives and for reading and writing data stored on a magnetic tape. Tape drives are sequential-access, and must wind past all preceding data to read any one particular piece of data.
Innovation in disk-based storage technology has reduced the density and cost per bit gap between disk and tape. Furthermore, the performance of a tape device cannot be guaranteed if the tape is dropped or is exposed to non-ideal environmental conditions.
The idea of emulating a tape storage system has been disclosed in WO 2005/017686 and WO 2005/033945. Using such emulation techniques a conventional tape library back-up system can be provided using disk arrays to replace tape cartridges as the physical back-up storage media. Physical tape-based media that would be present in a conventional tape library are replaced by what is termed herein as “virtual tapes.” A virtual tape library (VTL) therefore refers to an emulated tape-based library.
In a virtual tape device, capacity may be used on an ‘overcommitted’ basis. For example, a VTL may provide four virtual tape devices emulating second-generation Linear Tape-Open (LTO) media (known in the art as LTO-2 tape devices), whilst only having a disk capacity of 750 Gigabytes (GB). The actual recording capacity of four physical LTO-2 tape devices is 800 GB. In other words, the four emulated LTO-2 tape devices may appear to have a combined nominal capacity of 800 GB although the actual combined available capacity of the four emulated LTO-2 tape devices is only 750 GB. Thus, the amount of available disk space may run out before all of the virtual tape devices fill up.
Also, consider the example of four applications writing data continuously to a VTL of four LTO-2 tapes emulated on a disk-based system having a capacity of 750 GB. If writing to the virtual tapes simultaneously from empty, each of the virtual tapes will ideally attain 187.5 GB (750÷4) capacity.
However, now consider the same VTL having been written to by a single application from empty. In such a situation, there is sufficient disk-space available for the application to fill a first virtual LTO-2 tape with 200 GB of data. Having filled a first virtual LTO-2 tape with 200 GB of data, three further applications writing to the three remaining virtual tapes of the VTL should see virtual tapes having 183.3 GB ((750−200)÷3) capacity.
At present, emulated tape storage systems do not facilitate dynamic handling of changes in available disk capacity to maximize effectiveness of data storage in overcommitted systems.